A projectile weapon system includes equipment capable of measuring a desired firing direction. A firing direction is determined according to the angle (azimuth angle) of a target with respect to the north. In a hand-carried weapon system such as a mortar, a compass is used to measure an azimuth angle and is advantageous in that the azimuth angle may be estimated at relatively low cost, but the precision thereof is not high and the compass is influenced by the surrounding environment, and thus the precision is further deteriorated. In order to solve such a precision problem, an inertial navigation device may be used, but it is expensive and is unsuitable for hand-carried weapon systems due to a large weight and volume.
Further, it is possible to obtain an azimuth angle using an inertial navigation system such as a Global Positioning System (GPS), but an operation procedure for the system is very complicated, and difficulty in use is present in such a way that an additional satellite navigation receiver must be mounted at as long range as possible so as to obtain precision. Furthermore, this method is influenced by the surrounding environment such as geographic features or buildings, and is also influenced by intentional/unintentional electromagnetic environments.
Accordingly, various types of research into azimuth angle measurement techniques that meet small size/light weight/low cost required by hand-carried weapon systems such as mortars have been conducted in the past. Generally, an inertial navigation device is composed of three gyroscopes, wherein azimuth angle measurement equipment is configured using one or two gyroscopes so as to implement small-sized, lightweight, and inexpensive azimuth angle measurement equipment. However, a gyroscope required to obtain desired precision is generally expensive. Consequently, the azimuth angle measurement equipment is still expensive even if the price thereof is lower than that of the inertial navigation device. Therefore, research into a multi-position azimuth angle estimation technique and a rotation-type azimuth angle estimation technique using inexpensive medium-low level gyroscopes has been conducted.
Such a multi-position estimation technique is a method for eliminating a gyroscope bias that becomes the fundamental cause of error in an azimuth angle while changing the position of a gyroscope measurement axis, and thereafter estimating the azimuth angle. For such a technique, a 2-position estimation technique using two gyroscopes is well known. However, when 2-position estimation is performed using a single gyroscope, there are problems in that an inverse trigonometric function is a many-valued function and in that an error in the estimation of an azimuth angle is dependent on an actual azimuth angle, thus making it impossible to actually obtain an azimuth angle. Therefore, three or more positions are required, and an increase in the number of positions increases an estimation error due to a gyro random walk that is another error factor of a gyroscope. A rotation-type estimation technique is a method of rotating a gyroscope at constant velocity and eliminating a gyroscope bias using the output of the gyroscope and the rotational position/velocity information of the gyroscope. However, it is difficult to precisely configure rotation equipment, there is an environmental limitation, and cost is also increased.